<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    html,body {
      height: 100%;
    }
    .box {
      height: 100%;
      display: flex;
      flex-direction: column;
    }
    header {
      height: 46px;
      background-color: #ccc;
    }
    footer {
      height: 50px;
      background-color: #000;
    }
    main {
      /* 将剩余空间分配给main  */
      flex: 1;
      background-color: pink;
    }
  </style>
</head>
<body>
  <div class="box">
    <header>header</header>
    <main>main</main>
    <footer>footer</footer>
  </div>
</body>
</html>